(1) Field of the Invention
The present invention relates to a semiconductor laser device using a nitride semiconductor and a method for fabricating the same.
(2) Description of Related Art
Laser diodes emitting in the blue-violet region of the spectrum are expected as light sources for high-density optical disks, because the diameter of the focused laser spot on the optical disks can be reduced as compared with light diodes emitting in the red and infrared regions and therefore can improve the recording density for the optical disks. Semiconductor laser devices using nitride semiconductors, such as gallium nitride, have now been developed to realize laser diodes in the blue-violet region.
Devices of the following structure have been suggested as semiconductor laser devices using nitride semiconductors (see Japanese Unexamined Patent Publication No. 8-97507). FIG. 5 shows a cross-sectional structure of a semiconductor laser device according to a known art. As shown in FIG. 5, in a known semiconductor laser device, an n-type cladding layer 103 of n-type aluminum gallium nitride (AlGaN) is formed on a substrate 10 of sapphire with a low-temperature growth buffer layer 102 interposed between the substrate 101 and the n-type cladding layer 103. The following layers are successively stacked on the n-type cladding layer 103 to expose a part of the n-type cladding layer 103: an n-type guide layer 104 of n-type gallium nitride (GaN); an active layer 105 of indium gallium nitride (InGaN) having a multi-quantum well structure; a first p-type guide layer 106 of p-type GaN; a current blocking layer 108 of n-type AlGaN; a second p-type guide layer 107 of p-type GaN; a p-type cladding layer 109 of p-type AlGaN; and a p-type contact layer 110 of p-type GaN. Furthermore, an n-type electrode 111 is formed on an exposed part of the n-type cladding layer 103, and a p-type electrode 112 is formed on the p-type contact layer 110.
A part of the current blocking layer 108 is etched in a stripe, thereby forming a window through which current flows. When voltage is applied between the p-type electrode 112 and the n-type electrode 111, current flows only through the window obtained by removing the part of the current blocking layer 108. Therefore, current can be injected only into a stripe part of the active layer 105 located under the window. Furthermore, the refractive index difference between the current blocking layer 108 and the second p-type guide layer 107 permits the confinement of light emitted from the active layer 105 between the n-type guide layer 104 and the second p-type guide layer 107.
FIGS. 6A through 6C show a method for fabricating such a buried heterostructure semiconductor laser device. First, as shown in FIG. 6A, the first crystal growth is carried out in which a buffer layer 102, an n-type cladding layer 103, an n-type guide layer 104, an active layer 105, a first p-type guide layer 106, and a current blocking layer 108 are successively grown on a substrate 101.
Next, as shown in FIG. 6B, a part of the current blocking layer 108 is selectively removed by etching to form a stripe-like recess serving as a window.
Next, as shown in FIG. 6C, the second crystal growth is carried out in which a p-type guide layer 107, a p-type cladding layer 109 and a p-type contact layer 110 are successively formed on the current blocking layer 108 formed with the recess. Next, a p-type electrode and an n-type electrode are formed by a usual method.
Dry etching using a chloride gas is typically used to form the recess in the current blocking layer 108. If the recess is formed by dry etching, part of the current blocking layer 108 subjected to dry etching, such as an exposed part of the p-type guide layer 106, is damaged, resulting in the increased absorption of emitted light and the deteriorated device characteristics. Therefore, the recess is desirably formed by wet etching.
However, the buried heterostructure semiconductor laser device has the following problems. First, crystal growth need be carried out twice due to the structure of the device, and the second crystal growth need be carried out to the top surface of the current blocking layer 108 of n-type AlGaN except for the window. The regrowth of layers on the layer of AlGaN significantly worsens the surface morphology and crystallinity in early stages of the regrowth. This increases light absorption in part of the grown layers having deteriorated crystallinity, leading to the increased lasing threshold current.
Furthermore, when the p-type guide layer 107 of p-type GaN regrown on the current blocking layer 108 has deteriorated crystallinity, magnesium (Mg) with which the p-type guide layer 107 is doped is likely to diffuse through defects or the like into the active layer 105. When Mg has diffused into the active region 105, a semiconductor laser device using a nitride material has reduced reliability.
Moreover, the recess must be formed by dry etching causing damage to the current blocking layer 108 and other layers, because the current blocking layer 108 of n-type AlGaN can hardly be etched by known wet etching. The damage caused in a substrate region subjected to etching for the formation of the recess is one of factors responsible for the increased lasing threshold current.